The present invention relates to methods of making materials having a shape memory which is activated when the products are exposed to high humidity or moist environments.
Disposable absorbent products are typically subjected to one or more liquid insults, such as of water, urine, menses, or blood, during use. As such, the outer cover backsheet materials of the disposable absorbent products are typically made of liquid-insoluble and liquid impermeable materials, such as polypropylene films, that exhibit a sufficient strength and handling capability so that the disposable absorbent product retains its integrity during use by a wearer and does not allow leakage of the liquid insulting the product.
Although current disposable and reusable garments have been generally accepted by the public, these products still have the need of improvement in specific areas. For example, many absorbent products have a tendency to lose their shape or become uncomfortable to wear after the products are subjected to liquid insults and in-use conditions of high humidity and body temperature. Such changes in shape often result in ill fitting absorbent products and/or leakage.
Typically elastomeric materials are incorporated into disposable and reusable products to improve the fit of the products and prevent leakage during use. These elastomeric materials may be attached to the disposable product by several methods. At one time, elastic was applied to the substrate by sewing. (See U.S. Pat. No. 3,616,770 to Blyther et al.; and U.S. Pat. No. 2,509,674 and U.S. Pat. No. RE 22,038 to Cohen). A newer method for attaching elastomeric material to a substrate is by use of an adhesive. (See U.S. Pat. No. 3,860,003 to Buell.) Welding, such as sonic welding, has also been used to attach elastomeric material to a disposable product. (U.S. Pat. No. 3,560,292 to Butter). Laminates having an elastomeric layer and a co-extensive skin layer have also been used. (U.S. Pat. No. 5,429,856 to Kruger et al.).
However, these methods of attachment present several problems. First is the problem of how to keep the elastic in a stretched condition while applying the elastic to the substrate. Another problem is that attachment of a ribbon of elastomeric material will concentrate the elastomeric force in a relatively narrow line. This may cause the elastic to pinch and irritate the wearer""s skin. (See U.S. Pat. Nos. 3,860,003; 4,352,355; and 4,324,245 to Musek et al.; U.S. Pat. No. 4,239,578 to Gore; and U.S. Pat. Nos. 4,309,236 and 4,261,782 to Teed.) Other disadvantages of conventional attachment methods include speed, ease of manufacture, and cost. More importantly, difficulties may be encountered in maintaining a uniform tension on the elastic layer during its attachment to the substrate and also in handling the shirred article once the elastic layer is relaxed.
Heat-responsive elastomeric films overcome some of these detriments. Heat-responsive elastomers exist in two forms: a thermally-stable and a thermally-unstable form. The thermally-unstable form is created by stretching the material while heating near its crystalline or second phase transition temperature, followed by a rapid quenching to freeze it in the thermally-unstable, extended form. The elastomeric film can then be applied to a disposable product, for example a diaper, and heated to shirr or gather the elastomeric material, thereby producing a thermally-stable form of the elastomeric material. Examples of heat-responsive elastomeric films are disclosed in U.S. Pat. No. 4,681,580 to Reising et al., U.S. Pat. No. 4,710,189 to Lash, U.S. Pat. No. 3,819,401 to Massengale et al., U.S. Pat. No. 3,912,565 to Koch et al., and U.S. Pat. No. RE 28,688 to Cook.
These polymers have several disadvantages. The first of these disadvantages involves the temperature to which the elastomeric material must be heated to stretch the material to its thermally-unstable form. This temperature is an inherent property of the elastomeric material. Therefore, the disposable product is often difficult to engineer because temperatures useful for the production of the overall product may not be compatible with the temperature necessary to release the thermally-unstable form of the elastomer. Frequently, this temperature is rather high and can be detrimental to the adhesive material used to attach the various product layers. Another drawback to the use of heat-responsive elastomers is that they can constrain the manufacturing process, rendering it inflexible to lot variations, market availability, cost of raw materials, and customer demands.
U.S. Pat. No. 4,820,590 to Hodgkin et al. describes an elastomeric blend of three components to reduce the temperature required for the material to resume its heat stable form. Additionally, GB Patent 2,160,473 to Matray et al. proposes an elastomer which will shrink at an elevated temperature, for example at or above 170xc2x0 F. The advantageous features of these materials, compared to the heat-shrinkable materials discussed above, is that it does not require preheating during the stretching operation, but rather can be stretched at ambient temperatures by a differential speed roll process or by xe2x80x9ccold rolling.xe2x80x9d
Problems with use of these elastomers include difficulties inherent in applying a stretched elastic member to a flexible substrate such as a disposable diaper. Although some of the elastomers proposed have the advantage that they can be applied at ambient conditions in a highly stretched, unstable form, subsequent, often extreme, heating is required to release the thermally-unstable form to a contracted thermally-stable form. The temperature of this heat release is generally inflexible since it is determined at the molecular level of the elastomer. Thus, selection of materials for the disposable product which are compatible with this heating step is required.
Further, when individual heat activated elastic materials are used, the heat activation is generally accomplished by passing the garments through a heated air duct for a period of time. Since thermal heating must be transferred from an outer surface of the garment to inner portions of the garment, distribution of the activation means (i.e., thermal heat) throughout the garment takes considerable amounts of time and energy, resulting in an inefficient activation process. As a result, such heating processes can consume vast amounts of energy and undesirably result in slower manufacturing speeds.
What is needed in the art is a method of activating a shape deformation of a material without using an inefficient thermal heating activation process. What is also needed in the art is a method of activating a shape deformation of a material without substantially increasing the temperature of the material. Furthermore, there is a need for new materials that may be used in disposable absorbent products that generally retain their integrity during use, are easily disposed of, and have the ability to change to a desired shape and/or texture during in-use conditions. For example, upon exposure to a high humidity environment, the disposable product may transform to a desired product configuration which will guard against leakage.
The present invention addresses some of the difficulties and problems discussed above by providing a method of making a humidity responsive material having an amount of locked-in shape deformation. The method includes forming a latent deformation in the humidity responsive material. The material is capable of being deformed in at least one spatial dimension when exposed to one or more external forces, is capable of maintaining a degree of deformation in at least one spatial dimension once the external force is removed, and is capable of exhibiting a change, or percent recovery, in at least one spatial dimension when subjected to a humid or moist environment. The humid or moist environment may be created by in-use conditions of absorbent products.
More particularly, the latent deformation is formed by stretching the humidity responsive material. The stretched material possesses a draw or stretch ratio of at least 1.5 in one or more directions. In addition, the humidity responsive material is heated during stretching to a temperature of not more than about 120xc2x0 C.
Still more particularly, the humidity responsive material contains at least one shape deformable matrix material. The shape deformable matrix material may contain an elastomeric polymer and a non-elastomeric polymer, such as a moisture absorbing polymer. The humidity responsive material of the present invention finds applicability in a number of products, including products containing a gatherable or elastic part.
The present invention is further directed to a method of causing the shape deformation of materials having a desired amount of locked-in shape deformation. The method includes subjecting the material to a humid environment having a relative humidity of at least 50%. The method may be used to cause the shape deformation of the above-described material itself or a product containing the above-described material.
The present invention is further directed to a method of building shape deformable polymers in an effort to optimize the interaction of the shape deformable polymer with a selected level of humidity. By adjusting the chemical structure of the shape deformable polymer, one can tailor a specific shape deformable polymer in such a way as to maximize the interaction of the shape deformable polymer with a selected humidity level.
These and other features and advantages of the present invention will become apparent after a review of the following detailed description of the disclosed embodiments and the appended claims.